Color cathode ray tubes are, as shown in FIG. 8, have a bulb 8 formed by uniting a face plate 4 in which a fluorescent layer 2 is formed on the inside surface of a funnel 6. At the inside of a neck portion of bulb 8, there is received an electron gun 10 which emits R,G,B electron beams 12. The beams pass through apertures of a shadow mask 14 and then strike fluorescent layer 2 to form a picture element on a screen. A deflection yoke 16 disposed on the outside surface of the funnel 6 deflects the beams to form the picture on the screen.
The inline type electron gun for such color cathode ray tubes has advantages of being easily manufactured due to the simple structure in which R,G,B electron beams are transversely arranged in a line and omitting vertical dynamic convergence. However, this gun has the disadvantage of having serious spherical aberration of a focus lens for focusing the electron beams.
To reduce spherical aberration, it is well known that greater spacing between a pair of electrodes forming the focus lens creates an expanded focus lens. In this case, the greater the spacing between the pair of electrodes forming the focus lens, the smaller the spherical aberration is. However, when the spacing therebetween is excessively great, static electric charge generated from the periphery of the neck portion has influence on the focus lens to cause the electron beams misconvergence, which severely restricts the expanded focus lens of the inline type electron gun.
Thus, attempts at forming the expanded focus lens have been continued. One of them is described in U.S. Pat. No. 4,370,592 "Color Picture Tube having an Improved Inline electron Gun with an Expanded Focus Lens", issued to Richard H. Hughes on Jan. 25, 1983. FIG. 9 shows the main focus lens structure of the inline type electron gun provided in the U.S. Pat. No. 4,370,592. The electron gun of such method has R,G,B apertures 18R,18G, 18B, 20R, 20G, 20B which are included in a pair of electrodes 18,20, facing each other and has a horizontal lens 22,24 which is formed by deep drawing at a predetermined depth from the opposing side of the two electrodes 18 20 at their peripheries, thereby, in practice, forming the expanded main focus lens without the influence of external static charge. The width of both horizontal lenses 22,24 is, in practice, determined by the lateral width D of the third electrode 18 and by the lateral width E of the fourth electrode 20. While the lateral width D,E is severely restricted by the inside diameter of the neck portion of the color cathode ray tube (CRT), in a practical manufacturing process, it is much restricted by the distance A between the axes of each aperture. That is, while the horizontal lens 22,24 can be used in a SS(small separation) type of 5.08 mm or in a MS(middle separation) type of 5.5-5.6 mm, it can not be used in a LS(large separation) type of more than 6.6 mm.
The reason for not applying the horizontal lens to the LS type will be described as follows. FIG. 10 illustrates n a plan view the fourth electrode 20. Each R,G,B electron beam ER,EG,EB which passes through each aperture 20R,20G,20B thereof is positioned at the center thereof. However, when the distance A between the axes of each aperture is more than 6.6 mm, the respective distance from the two side apertures 20R,20B to the two ends of the horizontal lens 24 become different from each other, whereby each horizontal focusing voltage between the central aperture and the two side apertures is different.
The reason of the horizontal focusing voltage difference above is that the both side apertures 20R, 20B are nearer the electrode 20 than the central aperture 20G, whereby the side apertures have higher potential than the center aperture. For the same reason as above, the two side apertures 20R,20B have a partial or local difference between the horizontal and vertical focusing voltage, whereby the picture element formed on the screen is distorted.
FIG. 11 shows shapes of each electron beam when the expanded focus lens electron gun as described above is used in the LS type. As shown in this figure, a core C and a haro H of spot shape formed on the screen by R,G beams ER,EB are deformed right and left so as to be lopsided and in the spot shape of the center G beams EG, the lateral width is 130% of its vertical height for the core C to be formed in a lengthy ellipse shape, and, at the same time, an haro H is formed in a diamond type.